The long-term objective of the research is to determine the structure of DNA in chromosomes and to define molecular interactions stabilizing this structure. Investigations of bacterial chromosomes as well as those of Drosophila and Human cell lines are proposed. Specific experiments are intended to investigate the torsional tension in the DNA double-helix as it exists in chromosomes and chromatin in vivo. Methods using psoralen probes are employed to make these measurements. Changes in torsional tension during transcription or replication of specific genes will be investigated. It will be determined if the DNA in chromosomes and chromatin in their natural states in vivo is segregated into domains of supercoiling. The role of protein HU and other bacterial proteins in stabilizing negative DNA supercoils and in determining the amount of unrestrained torsional tension will be studied. Methods utilizing the genetics of interspecies cell hybrids will be developed to investigate the roles of specific non-histone chromosomal proteins in chromosome structure.